PC Hardware in a Nutshell, 3rd Edition

PCI Bus IRQ Steering

PCI Bus IRQ Steering is a function built into Windows 95 OSR2 or higher and Windows 98 (but not NT4 or Windows 2000/XP). IRQ Steering allows Windows itself to assign IRQs to PCI devices. With earlier versions of Windows 95, the BIOS assigns IRQs to PCI devices, and Windows must accept the decisions made by the BIOS IRQ Steering. If Windows IRQ Steering is enabled, Windows can override those BIOS decisions, although it seldom does so. OSR2 disables IRQ Steering by default; Windows 98 enables it by default.

Windows IRQ Steering allows Windows to reassign PCI interrupts automatically to accommodate the inflexible requirements of ISA devices. For example, assume that a PC with a BIOS that does not recognize non-Plug and Play ISA cards (that is, IRQ Steering is not implemented in BIOS) is running Windows 98 with IRQ Steering disabled. The PC is properly configured with all PCI devices, and the BIOS has assigned IRQ 11 to a Creative SoundBlaster AudioPCI 128 sound card. You then open the case and install a 3Com 3C509 network adapter (a non-Plug and Play ISA card), which is also configured for IRQ 11. When you restart the system, a conflict exists between the sound card on IRQ 11 and the network card, also on IRQ 11. If you enable Windows 98 IRQ Steering and restart the system, IRQ Steering takes the following actions during boot:

1. Detects that IRQ 11 is in use by both the PCI sound card and the ISA network card.

2. Disables the PCI sound card.

3. Maps a free IRQ—one that ...